Safety cut-off for electrical power operated hand tools



p 1967 H. cs. ALMQUIST 3,340,433

7 SAFETY CUT-QFF FOR ELECTRICAL POWER OPERATED HAND TOOLS Fild Feb. 15,1965 Rd 0 m to United States Patent 3,340,433 SAFETY CUT-OFF FORELECTRICAL POWER OPERATED HAND TOOLS Harry G. Almquist, 8128 S. WesternAve., Chicago, II]. 60620 Filed Feb. 15, 1965, Ser. No. 432,663 3Claims. (Cl. 317-18) This invention relates to an improved safetycut-off for electrical power operated hand tools.

One of the objects of my invention is the provision of means for use incombination with an electrical power operated hand tool used forcutting, drilling, or other working, which is operable to automaticallycut-off the power and arrest operation of the tool, as soon as it haspenetrated a layer of relatively non-electrically conducting materialand has contacted an underlying layer of relatively high conductingmaterial, so as to prevent injury both to the operator as well as to theunderlying layer.

Another object of my invention is the provision of a supervisoryelectrical circuit in combination with an electrical power operated toolfor cutting off power to the tool upon the occurrence of unsafeconditions under which it is desired to arrest operation of the tool.

A further object of my invention is the provision of means of theforegoing character which is simple in construction, effective inoperation and economical to manufacture.

Other and further objects and advantages of this invention will becomeapparent from the following description when the same is considered inconnection with the accompanying drawing, in which the single figure ofthe drawing illustrates a schematic representation of an electricalcircuit according to my invention, shown in conjunction with aconventional electric drill, the latter being illustrated as drillingthrough a non-electrical conducting material underlying which is anelectrical conductor within a conduit.

Referring to the drawing, the electrical tool for purposes ofrepresentation is shown as a conventional electrical drill 5. Thesupervisory electrical circuit is preferably contained in a housing, notshown. Power for the circuit and for the tool is derived through a threewire male plug, not shown, which is adapted to be received in and makecontact with a conventional 120 volt AC receptacle or other alternatingcurrent supply of higher voltage, depending upon the characteristics ofthe tool used. Three conductors running from the plug consist of hotwire 6, neutral wire 7 and building ground wire 8.

Control power for the circuit is derived through stepdown transformer 9,the primary winding .11 of which is connected across wires 6 and 7 andthe secondary winding of which is connected at one side to ground wire 8and reset switch 13, the other side being connected to an intermediatepoint of the coil of relay 12. The transformer 9 is adapted to step-downthe supply current to 24 V. AC. on the secondary winding terminals.

The relay 12, illustrated, has a split coil design with one set of relaycontacts 15. One half of the coil is operable to close the line voltagecircuit and the other half to open it. A suitable construction of relay,as herein described, is General Electric Catalog No. RR-3 which operateson a 24 volt switch circuit for controlling a 125, 250 or 277 voltcircuit.

Reset switch 13 also is connected in circuit with a terminal of thesplit coil of relay 12, the other terminal of the split coil of relay 12being grounded to the housing of drill 5 through conductor 19. The powerfor drill 5 is derived through conductors :16 and 17 connectedrespectively to power lines 67, with conductor 16 being connected torelay contacts 15.

When the drill 5 is not operating, the relay contacts 15 are in theposition illustrated in the drawing. In order to render the drill 5operative, reset switch .13 is momentarily depressed to close thecircuit through secondary winding 10 and the left one-half of relay coil12, thereby energizing said coil and moving the armature A towardscontacts 15 and establishing current flow across said contacts. Afterthe reset switch 13 is released the armature A is mechanicallymaintained in circuit closing relationship with contacts 15. When theswitch connected to the motor of drill 5 is actuated, a circuit iscompleted between the power lines 6 and 7 and drill 5 through conductors16 and 17 thereby effecting operation of the drill.

In the event that the bit of the drill 5 makes grounding contact with aconcealed pipe, electric conduit, or the like, the right one-half ofrelay coil 12 is immediately energized by current flow through it fromthe secondary winding 10 of the transformer, through conductor 19,through the drill bit, grounded contact C and through conductor 8. As aresult of such energization, armature A which has been mechanically heldin circuit closing condition is caused to move to the left, disengagingrelay contacts 15 and opening the circuit to the motor of drill 5,causing the same to be stopped. Further operation of the drill isrendered impossible until reset switch 13 is again actuated. It willthus be seen that injury both to the operator and to the tool is therebyavoided.

It will be understood that various changes and modifications may be madefrom the foregoing without departing from the spirit and scope of theappended claims.

I claim:

1. A safety cut-off means comprising, in combination, an electricalpower driven hand tool, a current source, a split coil relay having apair of relay contacts, a power circuit for said tool including onlysaid relay contacts in series with said tool, a step-down transformerthe primary of which is connected across said current source, one sideof the secondary of said transformer being connected intermediate saidsplit relay coil, the other of said secondary being connected to oneside of said relay coil and to a ground lead, the other side of saidrelay coil being connected to the housing of said too-l, one-half ofsaid relay coil when energized effecting closing of said contacts toclose the power circuit to said tool, the other one-half of said relaycoil being energized upon the housing of said tool being groundedwhereby said contacts are opened thereby cutting off power and stoppingoperation of said tool.

2. The invention as defined in claim 1 in which a normally opened resetswitch is connected in the line between said other side of saidsecondary and said one side of said relay.

3. A safety cut-oif means comprising, in combination, an electric powerdriven hand tool, a source of electric current, a first electric circuitconnected to said source affording power for operating said tool, asplit coil relay,

said circuit including only a pair of relay contacts in series with saidtool, a second electric circuit connected to said source and includingthe operating coil of said relay, means in said second circuit forenergizing one-half of said coil to efiect closing of said contacts tocomplete said first electric circuit, and means in said second electriccircuit for energizing said second one-half coil to open said contactsresponsive to conditions under which it is desired to stop operation ofthe tool.

References Cited FOREIGN PATENTS 839,744 1/ 1939 France. 634,602 9/ 1930Great Britain. 551,090 2/ 1943 Great Britain.

MILTON O. HIRSHFII'EIJD, Primary Examiner.

R. V. LUPO, Assistant Examiner.

1. A SAFETY CUT-OFF MEANS COMPRISING, IN COMBINATION AN ELECTRICAL POWER DRIVEN HAND TOOL, A CURRENT SOURCE, A SPLIT COIL RELAY HAVING A PAIR OF RELAY CONTACTS, A POWER CIRCUIT FOR SAID TOOL INCLUDING ONLY SAID RELAY CONTACTS IN SERIES WITH SAID TOOL, A STEP-DOWN TRANSFORMER THE PRIMARY OF WHICH IS CONNECTED ACROSS SAID CURRENT SOURCE, ONE SIDE OF THE SECONDARY OF SAID TRANSFORMER BEING CONNECTED INTERMEDIATE SAID SPLIT RELAY COIL, THE OTHER OF SAID SECONDARY BEING CONNECTED TO ONE SIDE OF SAID RELAY COIL AND TO A GROUND LEAD, THE OTHER SIDE OF SAID RELAY COIL BEING CONNECTED TO THE HOUSING OF SAID TOOL, ONE-HALF OF SAID RELAY COIL WHEN ENERGIZED EFFECTING CLOSING OF SAID CONTACTS TO CLOSE THE POWER CIRCUIT TO SAID TOOL, THE OTHER ONE-HALF OF SAID RELAY COIL BEING ENERGIZED UPON THE HOUSING OF SAID TOOL BEING GROUNDED WHEREBY SAID CONTACTS ARE OPENED THEREBY CUTTING OFF POWER AND STOPPING OPERATION OF SAID TOOL. 